1. Field of the Invention
The invention relates generally to solid state lasers, and more particularly to solid state laser materials.
2. Description of Related Art
Fluoride and oxide crystals doped with transition metal ions have long been known to serve usefully as laser materials. The first laser was a flashlamp-pumped ruby, which is Cr.sup.3+ -doped Al.sub.2 O.sub.3 [Maiman, "Stimulated optical radiation in ruby," Nature 187, 493 (1960)]. The Cr.sup.3+ dopants color the Al.sub.2 O.sub.3 crystals (sapphire) pink or red, and are responsible for the laser activity of the medium. While the ruby laser once aroused much interest and exploration, it ultimately proved to be of limited utility. One limitation of the material is its narrow gain spectrum (centered at 692 nm); a broader gain spectrum would prove to be more useful.
The first broadly tunable gain media, based on the transition metal ions Ni.sup.2+, Co.sup.2+ and V.sup.2+, were discovered by L. Johnson and coworkers [L. F. Johnson et al., "Phonon terminated optical masers," Phys. Rev. 149, 179 (1966); also Auzel et al., U.S. Pat. No. 4,627,064]. Again, these lasers did not turn out to be of substantial practical interest since several fundamental issues limit their utility. For example, both Ni.sup.2+ and V.sup.2+ -based lasers tend to operate with low efficiency [S. A. Payne et al., "Excited state absorption spectra of V.sup.2+ in KMgF.sub.3 and MgF.sub.2," Phys. Rev. B 37, 998 (1988); S. A. Payne, "Energy level assignments for .sup.1 E and .sup.3 T.sub.1a states of MgO:Ni.sup.2+," Phys. Rev. B 41, 6109 (1990)], because of interference from excited state absorption and due to losses arising from nonradiative decay. Excited state absorption (ESA) is a process where light at the laser wavelength experiences absorption from the excited state to a higher lying excited state, thereby inducing loss rather than gain (where the excited state is stimulated to radiate back to the ground state). Nonradiative decay is also detrimental to the operation of the laser since the energy stored in the excited states that are produced lead to heat generation, rather than to light (and therefore to gain). Co.sup.2+ -based lasers such as Co:MgF.sub.2 experience low ESA loss although the nonradiative decay is substantial [D. M. Rines et al., "High energy operation of a Co:MgF.sub.2 laser," Opt. Lett. 19, 628 (1994)]. Rines et al. has shown that the Co:MgF.sub.2 laser can operate efficiently if it is pumped with a .about.50 .mu.sec pulse from a 1.3 .mu.m Nd:YAG laser, although the overall efficiency of this laser-pumped laser system remains low.
Lasers based on the Cr.sup.3+ ion have proved to be the most useful laser systems. Although Cr.sup.3+ tends to incur little nonradiative decay at room temperature in most oxide and fluoride crystal hosts, the ESA losses are found to vary significantly depending on the particular host medium [Caird et al., "Quantum electronic properties of the Na.sub.3 Ga.sub.2 Li.sub.3 F.sub.12 :Cr.sup.3+ laser," IEEE J. Quantum Electron. 24, 1077 (1988)]. Examples of Cr.sup.3+ -lasers include alexandrite (Walling et al., U.S. Pat. No. 4,272,733 and Morris et al., U.S. Pat. Re 31,057), Cr:SrAlF.sub.5 (Jenssen, U.S. Pat. No. 4,599,727), Cr:Y.sub.3 Ga.sub.5 O.sub.12 (Walling et al., U.S. Pat. No. 4,490,822), emerald (Alfano et al., U.S. Pat. No. 4,464,761), Cr-doped germanate oxides (McCollum et al., U.S. Pat. No. 4,648,094), Cr:ScBO.sub.3 (Chai et al., U.S. Pat. No. 4,841,530), Cr-doped elpasolite crystals (McCollum et al., U.S. Pat. No. 4,377,864), and Cr-doped LiCaAlF.sub.6 and LiSrAlF.sub.6 (Cr:LiSAF) [Payne et al., U.S. Pat. No. 4,811,349 and Krupke et al., U.S. Pat. No. 5,105,434]. Of these Cr-lasers, only alexandrite and Cr:LiSAF are routinely employed in applications. The Ti.sup.3+ transition metal ion has also been found to offer useful laser properties in the Al.sub.2 O.sub.3 (sapphire) host [P. F. Moulton, "Spectroscopic and laser characteristics of Ti:Al.sub.2 O.sub.3," J. Opt. Soc. Am. B 3, 125 (1986)]. Ti:sapphire lasers are efficient and broadly tunable from 0.7-1.1 .mu.m.
All of the examples cited above entail crystals where the transition metal ion is coordinated by six fluorines or six oxygens in an octahedral site. While the known laser materials based on transition metal ions overwhelmingly involve octahedral coordination of the laser ions, there are a few examples where tetrahedral, or four-fold coordinated, sites are employed. The main transition metal ions of this nature include Cr.sup.4+, and to a lesser extent, Mn.sup.5+ (Alfano, U.S. Pat. No. 4,932,031 and Alfano, U.S. Pat. No. 4,987,575). The present invention is based on transition metal ions that are incorporated onto tetrahedral sites.
Bishop et al. disclose lasers based on transition metal ions doped into III-V compounds, and cite a particular embodiment as involving Fe in InP [Bishop et al., U.S. Pat. No. 4,158,207]. The III-V notation refers to the number of valence electrons in the neutral state of the cation (e.g., In with three electrons) and for the anion (e.g., P with five electrons). In contrast, the present invention involves so-called II-VI compounds such as ZnS.
Transition metal ion dopants in II-VI compounds emitting in the visible spectral range have previously been recognized to serve usefully in fluorescence display materials (e.g., Goede et al., German Patent No. DD 251,153; Mullermach et al., German Patent No. DD 255,429; and Japanese Patent Nos. JP 56088487 and JP 85008073). In these cases, the transition metal doped II-VI compounds are considered for electroluminescence behavior rather than as laser materials.
The scientific literature contains many references to the optical properties of transition metal ions in II-VI compounds although their potential for laser action was never recognized. Weakliem's classic paper from 1962 reports the absorption spectra of Ni.sup.2+, Co.sup.2+ and Cu.sup.2+ in crystals such as ZnO, ZnS and CdS [Weakliem, "Optical spectra of Ni.sup.2+, Co.sup.2+ and Cu.sup.2+ in tetrahedral sites in crystals," J. Chem. Phys. 36, 2117 (1962)]. Weakliem investigates the nature of the energy levels, establishes their electronic symmetries, and provides a theoretical basis for their absorption strength. Other examples of related studies include: Vallin et al., "Infrared absorption in some II-VI compounds doped with Cr," Phys Rev. B 2, 4313 (1970); and Pappalardo et al., "Absorption spectra of transition ions in CdS crystals," Phys. Rev. 123, 1188 (1961). Furthermore, the scientific literature contains numerous reports of the infrared luminescences arising from II-VI compounds doped with transition ions. Since the researchers' main objective involved the exploration of fundamental physics issues, most of the luminescence data was recorded below room temperature where the spectra can be better resolved. Some previously-reported data include: Kaminska, "Absorption and luminescence of Cr.sup.2+ (d.sup.4) in II-VI compounds," J. Phys. C: Solid State 12, 2197 (1979); Radlinski, "Infrared luminescence of cobalt impurities in II-VI compounds," J. Luminescence 18/19, 147 (1979); and Slack et al., "Infrared luminescence of Fe.sup.2+ in ZnS," Phys. Rev. 163,335 (1967). Additional work on other types of transition metal-doped sulfides have been reported, such as ZnGa.sub.2 S.sub.4 [H. Park et al., "Transition behavior of Mn.sup.2+ - and Ni.sup.2+ -doped ZnGa.sub.2 S.sub.4 crystals," Phys. Stat. Sol. B 180, K69 (1993)]. Finally, the observed emission lifetime is a useful quantity from which to judge the potential for laser action, since in some circumstances it may be employed to calculate the emission cross section. Moreover, when the emission lifetime is recorded as a function of temperature, an abrupt decrease in the emission lifetime with rising temperature may be interpreted as the onset of nonradiative decay (i.e., the decrease in luminescence yield). Several papers have appeared on this topic, including: Dubenski, "Luminescence and absorption associated with the lower terms of Co.sup.2+ and Ni.sup.2+ ions in single crystal ZnS," Optics and Spectroscopy 19, 353 (1965); Renz et al., "Temperature dependence of the lifetime of excited states for 3d transition element centres in II-VI crystals," J. Luminescence 24/25, 221 (1981); and Radlinski, "Position of the Co.sup.2+ level in wide-gap II-VI semiconductors," J. Phys. C: Solid State 12, 4477 (1979).
The survey of the scientific literature described above has been combined and reinterpreted in the present invention to predict that transition metal-doped II-VI compounds can serve as lasers. These basic spectroscopy studies have been pursued for more than three decades now without any previous indication that they are active laser media. This occurrence is not surprising, in that laser action requires that many factors be favorable, such as the existence of low ESA losses and high luminescence efficiency (low nonradiative decay); the availability of high optical quality material with the required doping level; and adequate pump sources. The present invention brings all the factors together to produce a new class of laser materials and lasers.